Spirally wound membrane modules and apparatus containing such modules have been employed in connection with the reverse osmosis and ultrafiltration separation of feed streams to a concentrate stream and permeate stream. For example, spirally wound membrane modules are often employed in ultrafiltration processes for separating food products, such as whey streams and gelatin streams or for the separation of fruit juices and concentration of aqueous electrodeposition paints, while spirally wound membrane modules are also employed for other purposes including reverse osmosis processes.
Spirally wound membrane modules typically comprise a central tube having passageways therein, a spirally wound membrane leaf composed of a pair of membrane sheets with a solid permeate collection sheet interposed therebetween and the membrane sheets sealed around the edges and wound around a central tube. The membrane leaf is spirally wound around the central tube with the membrane leaves separated by a mesh-type spacer material which forms a feed channel for the introduction of a feed stream in the channel created by the spacer and generally axially with the central tube and across the spacer material. The feed channel spacer mesh material typically comprises a type of plastic mesh material in which single filaments of plastic material are crisscrossed to form a diamond, net-like pattern.
In operation, the spirally wound membrane module is disposed in a housing and generally a feed solution to be separated is introduced into one end of the housing so that the feed solution flows generally axially across and in the feed spacer channel created by the spacer material, and a permeate stream is withdrawn through permeate collection holes in the central tube and from one end of the housing, while at the other end of the spirally wound membrane module in the housing, a concentrate stream is withdrawn. The feed stream flows across the feed spacer channel, and the permeate is collected within the membrane leaf material directed generally spirally and radially toward the permeate collection holes in the central tube while the concentrate material is removed from the other end of the housing of the spiral membrane module. Generally, in operation, the spiral membrane apparatus are employed in parallel or series.
A spiral membrane module with a controlled bypass seal is disclosed for example in U.S. Pat. No. 4,301,013, issued Nov. 17, 1981 and wherein the controlled bypass of the feed stream around and in the annular space about the exterior of the spirally wound membrane module and housing is provided by the use of a heat-shrinkable, open-type mesh material in the annular clearance space. The open mesh material permits control of the bypass about the annular clearance space to control the angle of the strands of fibers of the mesh material employed. The strands are placed at an angle to the axial flow, and such open mesh material is generally used also to form the mesh spacer material as feed channel spacer between the spirally wound membrane leaves.